The present invention relates to a magneto-optical disk, and more particularly to a magneto-optical disk with protective coatings for providing anti-reflection and anti-oxidation for the disk.
A known magneto-optical disk generally comprises a substrate made of polycarbonate, a recording layer coated on the polycarbonate substrate, and at least one protective coating adapted for providing anti-reflection and anti-oxidation for the recording layer.
Referring to FIG. 1 which is a cross-sectional view illustrating a construction of an embodiment of the known magneto-optical disk, the known magneto-optical disk comprises a polycarbonate substrate 11, a first dielectric layer 12 of an optimal thickness (about 800 .ANG.), as a protective coating, coated on the polycarbonate substrate 1, a recording layer 13 of an alloy from the amorphous RE-TM alloy (Rare Earth-Transition Metal system, TbFeCo alloy in this embodiment) coated on the first dielectric layer 12, a second dielectric layer of an optimal thickness (about 400 .ANG.), as a protective coating, coated on the recording layer 13, and a reflective layer 15 of Al, Cr or Ti coated on the second dielectric layer 14.
In the known magneto-optical disk having the above-mentioned construction, the laser beam transmitted through the polycarbonate substrate 1 reaches to the recording layer 13 by way of the first dielectric layer 12 in order for a part thereof to be reflected on the surface of the recording layer 13, thereby causing the Kerr rotation angle .theta.k and the Faraday effect .theta.f are much influenced by refractive indexes, transmissivities and thicknesses of the first and second dielectric layers 12 and 14, for example, the .theta.k and .theta.f increase in proportion to the refractive indexes.
Each above-mentioned dielectric layer has been generally formed of SiC as carbide, SiN.sub.x and AlN as nitride, SiO.sub.2, SiO and TiO.sub.2 as oxide, etc . . . where each formula represents the mole ratios of the atoms. Of these dielectric materials, TiO.sub.2 has not been used because it might deteriorate the quality of the recording layer and had a liability as a result of its reactivity with the oxygen in atmosphere, while it might provide a high anti-reflection effect for the recording layer, resulting from its relatively higher refractive index ranging from 2 to 3. Therefore, there has been generally used a dielectric material from SiNx, especially Si.sub.3 N.sub.4, in spite of its relatively lower refractive index ranging from 1.5 to 2.0, resulting in inducing relatively lower .theta.k and .theta.f. For instance, it is well known that the maximum .theta.k of 1.2.degree. appears when the thickness of the SiNx layer is 800 .ANG., which .theta.k value of 1.2.degree. is deficient to provide a desired anti-reflection effect for the optical disk.
According to the above-mentioned description of the prior art, it is well noted that there is a serious drawback that the known magneto-optical disk appears a seriously low anti-reflection effect.